This invention relates in general to vehicle hydraulic brake systems and in particular to an electronic rear brake proportioning system.
Most non-commercial vehicles and light trucks are equipped with hydraulic brake systems having a dual master cylinder which includes separate front and rear hydraulic brake fluid reservoirs and actuating chambers. The front and rear actuating chambers are connected to front and rear brake cylinders, respectively, to define separate front and rear brake circuits. Depressing a brake pedal, which is connected by a mechanical linkage to the master cylinder, applies hydraulic pressure through both brake circuits to the brake cylinders at each of the vehicle wheels. The brake cylinders actuate the front and rear wheel brakes to slow the vehicle. By dividing the brake system into front and rear brake circuits, braking capability is maintained if a brake fluid leak should develop in one of the brake circuits.
During a braking cycle, a portion of the weight of a vehicle is transferred from the rear vehicle wheels to the front vehicle wheels. The transfer of weight increases the frictional force produced between the front vehicle wheels and the road surface while decreasing the frictional force produced between the rear vehicle wheels and the road surface. Accordingly, if an equal braking force is applied to the front and rear vehicle wheels, the transfer of vehicle weight between the rear and front wheels could cause the rear wheels to lock up while the front wheels continue to rotate. A vehicle with locked rear wheels and rotating front wheels could spin-out easily.
To maintain directional stability during a braking cycle, the front wheels must lock up before the rear wheels. Thus, it is known to equip vehicles with disc brakes on the front wheels and drum brakes on the rear wheels or to provide smaller calipers on rear disc brakes than on front disc brakes. The use of more efficient brakes on the front wheels results in a greater braking force being applied to the front wheels than the rear wheels when the same hydraulic pressure is applied to the front and rear brake cylinders.
On a low mu road surface, little or no weight is transferred from the rear to the front of a vehicle during a braking cycle. Thus, the more efficient design of the front brakes is usually sufficient on low mu surfaces to assure that the front wheel brakes lock up before the rear wheel brakes, thereby preserving vehicle directional stability.
On high mu surface roads, however, the greater coefficient of friction of the road surface allows a harder brake application with a corresponding greater vehicle deceleration. The increase in vehicle deceleration results in a transfer of vehicle weight from the rear wheels to the front wheels. With the transfer of vehicle weight, the design of the brake calipers may not be sufficient to assure that the front brakes lock up before the rear brakes. Accordingly, a proportioning valve is typically included in the rear brake circuit.
The proportioning valve is operative to increase the hydraulic pressure applied to the rear wheel brake cylinders at a slower rate than the rate of increase of the hydraulic pressure applied to the front wheel brake cylinders. The different rates of increase result in a greater braking effort occurring at the front wheel brakes than at the rear wheel brakes such that the front wheel brakes lock up before the rear wheel brakes. The proportioning valve is operative only after a predetermined brake pressure threshold has been exceeded. The predetermined brake pressure threshold is commonly referred to as the changeover pressure. For hydraulic pressures below the changeover pressure, the same hydraulic pressure is applied to both the front and rear brake cylinders.
Some vehicles have a diagonally split brake system with the left front and right rear wheel brakes actuated by one actuating chamber of the master cylinder and the right front and left rear wheel brakes actuated by the other actuating chamber of the master cylinder. Thus, if one of the brake circuits should fail, the other brake circuit remains operational to apply braking force at both ends of the vehicle. Such diagonally split brake systems require two proportioning valves, with one proportioning valve included between the master cylinder and each rear wheel brake cylinder.
This invention relates to an electronic rear brake proportioning system.
As described above, a proportioning valve is typically included in a vehicle brake system to assure that the front wheels will lock up before the rear wheels on a high mu surface. As also described above, on low mu surfaces, the brake pressure seldom exceeds the proportioning valve threshold. Accordingly, the proportioning valve is typically actuated only on high mu surfaces. A unique proportioning valve is calibrated for each particular vehicle to accommodate the vehicle""s specific design characteristics to provide an optimum braking response.
It would be desirable if one universal proportioning valve could be provided for different vehicles. Also, for diagonally split systems, it would be desirable to reduce the required number of proportioning valves.
The present invention contemplates an electronic rear brake proportioning system for a vehicle having at least one front wheel brake and one rear wheel brake, the wheel brakes being coupled to and actuated by a master cylinder. The electronic rear brake proportioning system includes a normally open solenoid valve connected between the master cylinder and the rear wheel brake and a controller electrically connected to the solenoid valve The controller is enabled when a first vehicle operating parameter exceeds a first threshold and the controller is operable to close the solenoid valve to isolate the rear wheel brake from the master cylinder when a second vehicle operating parameter exceeds a second threshold.
The system also includes a speed sensor for monitoring the speed of the wheel associated with the rear wheel brake. The speed sensor generates a rear wheel speed signal. The invention also contemplates that the first vehicle operating parameter is the rear wheel speed and that the first threshold is a predetermined rear wheel speed. The controller being enabled when the rear wheel speed exceeds the predetermined rear wheel speed.
It is further contemplated that the controller is operative to calculate a rear wheel slip. In the preferred embodiment, the second vehicle operating parameter is rear wheel slip and the second threshold includes a predetermined value of rear wheel slip. Accordingly, the controller closes the solenoid valve upon being enabled and the rear wheel slip exceeding the rear wheel slip threshold.
The controller also can be operative to calculate a deceleration of the controlled rear wheel. The rear wheel deceleration can the be utilized to calculate a road surface factor which can be combined with the predetermined value of rear wheel slip to define the rear wheel slip threshold. Additionally, the controller can calculate a deceleration factor which is a function of elapsed braking time. The deceleration factor can be combined with the road surface factor and the predetermined value of rear wheel slip to further refine the rear wheel slip threshold.
The present invention also contemplates that the controller is further operable to selectively and intermittently reopen the solenoid valve to increase the pressure applied to the controlled rear wheel brake. Additionally, the controller can be operative to maintain said isolation of the rear wheel brake for a predetermined time period after the vehicle has stopped. Furthermore, the controller can be responsive to a vehicle brake pedal being released to open the solenoid valve after a shortened predetermined time period has elapsed. The controller also can be operative to open the solenoid valve upon the elapse of a predetermined time period following the rear wheel slip falling below the rear wheel slip threshold. Similarly, the controller can be operative to open the solenoid valve upon the elapse of a predetermined time period following the rear wheel speed exceeding a predetermined rear wheel speed recovery threshold.
It also is contemplated that the rear brake proportioning system can be included in an anti-lock brake system or a traction control system.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.